Process for the production of synthetic diopside

ABSTRACT

A process for the production of synthetic diopside wherein dolomite and silicic acid are hydrothermally converted into calcium and magnesium hydrosilicates which are in turn converted by dehydration into diopside. In the hydrothermal conversion from 10 to 40 percent by weight of water is added to the dolomite and silicic acid in a closed reaction vessel, and the reaction mixture is preferably simultaneously mixed and comminuted. The resulting calcium and magnesium hydrosilicates are preferably heated to a temperature of 1,200* to 1,250* C. and are converted into diopside.

United States Patent Wuhrer et al. {451 Mar. 28, 1972 [54] PROCESS FORTHE PRODUCTION OF 2,590,566 3/1952 Osborn ..23/110 R SYNTHETIC DIOPSIDE3,257,220 6/1966 Kalousek et a1... 2,665,996 1/1954 Kalousek [72]Inventors: Josef Wuhrer, Wulfrath; Heinz-Georg 3 033 4 5/19 2 Vandal-Linden Kumyk, Monchensladbach; Adrian 2,343,151 2/1944 Maclntire..23/201 tenheym, Wulfrath, all of Germany 73 Assignee: 11116111156116Kalksteinwerke GmbH, Wul- Primary i' stem frath, Germany Attomey-Chnsten& Sabol [22] Filed: Apr. 10, 1970 [57] ABSTRACT [21] Appl. No.: 27,434 Aprocess for the production of synthetic diopside wherein dolomite andsilicic acid are hydrothennally convened into calcium and magnesiumhydrosilicates which are in turn con- [30] Foreign Appuufion Pnomy Damverted by dehydration into diopside. 1n the hydrothermal con- Apr. 11,1969 Germany ..P 19 18 459.9 version from 10 to 40 percent by weight ofwater is added to the dolomite and silicic acid in a closed reactionvessel, and

[52] U.S. Cl. ..23/110 R, 23/182, 106/120 the reaction mixture ispreferably simultaneously mixed and [51] ..C01b33/22, C01b 33/24comminuted. The resulting calcium and magnesium hydrosil- [58] Field ofSearch ..23/110, 182,201; 106/120 icates are preferably heated to atemperature of 1,200 to 1,250 C. and are converted into diopside.

[56] References Cited 5 Claims, No Drawings UNITED STATES PATENTS3,264,130 8/1966 Mays 237119195 W PROCESS FOR THE PRODUCTION OFSYNTHETIC DIOPSIDE The present invention relates to a process for theproduction of synthetic diopside CaMgSi,O from dolomite and silicicacid.

Diopside is the silicate of calcium and magnesium and is a valuable rawmaterial for the ceramic industry. Naturally occurring diopside is amonoclinic pyroxene and ideally consists of the silicate of calcium andmagnesium CaMgSi,O Commonly, however, the diopside contains a variablecontent of Fesi o, in addition thereto and is thus not desirable forceramic purposes. Moreover, diopside is not found in large quantities innature, occurring usually in small quantities'in company withwollastonite (CaOSiO,). Diopside, however, is a valuable ceramic rawmaterial since it is particularly suitable for use in formation ofelectric insulators because it makes possible low firing temperatures.The heat resistance of diopside is higher when compared for example tothe customarily used enstatite (a magnesium silicate).

Heretofore in the ceramics industry it has been known to use a mixtureof wollastonite as a calcium silicate source, and a basic substancesimilar to enstatite as a magnesium silicate source, to approximate thenaturally occurring diopside. The enstatite-like substance isconventionally obtained by firing of steatite, a coarse, massive orgranular variety of talc and thus an acid metasilicate of magnesium.Steatite has the approximate composition 3MgO-4SiO,-H,O to4MgO-5SiO,-H,O and in addition contains from 5 to percent by weightimpurities. During heating of the steatite to higher temperatures in thefiring process, the magnesium metasilicates enstatite or clinoenstatite(MgO-Si0 and cristobalite (crystalline silica) are formed. The presenceof cristobalite in the product, however, is highly undesirable since itdecreases heat resistance of products made therefrom. There hastherefore remained the task of developing an economic process for theproduction of synthetic diopside.

it is therefore an object of the present invention to provide a novelprocess for the production of calcium and magnesium hydrosilicates, andespecially of Xonotlite and Cerolite.

it is a further object of the present invention to provide a process forthe production of synthetic diopside.

It is another object of the present invention to provide such processeswhich are economical and which overcome the disadvantages of prior artprocesses.

It is still another object of the present invention to provide asynthetic diopside which is highly advantageous over prior artapproximations of the naturally occurring mineral in the ceramicsindustry.

Other objects and advantages of the present invention will becomeapparent from a consideration of the following specification and claims.For a more comprehensive disclosure of the nature, objects andadvantages of the invention, reference is made to the following detaileddescription of the process of the present invention for the productionof calcium and magnesium hydrosilicates and for the subsequent preferredproduction of synthetic diopside.

ln copending application Ser. No. 859,407 filed Sept. 19, 1969, it isdisclosed that synthetic wollastonite CaOSiO, is obtained from lime andsilicic acid. In that process, the lime and the silicic acid are firstconverted in approximately equivalent quantities and in presence ofwater to calcium hydrosilicates, which subsequently are dehydrated intowollastonite. According to a preferred form of the process, thehydrothermal conversion to calcium hydrosilicates is carried out attemperatures of from 170 to 200 C., and it is advantageous to carry outthis conversion with simultaneous crushing or grinding of the reactionmixture.

It has now been found that calcium and magnesium hydrosilicates,especially Xonotlite and Cerolite, can be produced through thehydrothermal conversion of dolomite and silicic acid in approximatelyequivalent quantities at elevated temperatures and under pressure, whenthe starting substances of dolomite and silicic acid are mixed graduallywith 10-40 percent by weight of water, based on the weight of thestarting substances, in a closed vessel, whereby temperatures above 170C. and preferably above 200 C, occur as a result of the hydration heatof the CaO component of the dolomite. Preferably the closed vessel is agrinding machine and as the hydrothermal conversion proceeds, thestarting substances and also the reaction products are simultaneouslycomminuted in the grinding machine. Surprisingly, as a result of thisprocess, almost complete conversions into calcium and magnesiumhydrosilicates occur within very short reaction times. Depending on thequantity of water used, an almost dry to briquettably moist mass can beobtained as an end-product.

Following the initial formation of the calcium hydrosilicates, acatalytically controlled avalanche-like process is released. During thisreaction, it is assumed that initially phases similar to tober'morite(Ca [Si O (OH),] [Ca(Ol-l), ],,'4H,O) will develop, which convert intoXonotlite (calcium hydrosilicate) while releasing water, which againparticipates in the formation of tobermorite phases. In this manner, thesmall quantities of water according to the present invention aresufficient and this fact has the advantage that formation of the calciumhydrosilicate, which is an exothermic reaction, makes an outside energysupply unnecessary for maintenance of the reaction temperature becauseof the steam pressure developed thereby. However, it is important thatfor continued formation of the hydrosilicates there be present both freewater and a steam saturated atmosphere in the reaction vessel. For theinitiation of the reaction between the CaO component of the dolomite andsilicic acid, a certain quantity of water is necessary. This quantity ofwater slakes a part of the CaO while forming calcium hydroxide andsimultaneously forming steam. As a result, conditions are created forthe formation of tobennorite phases, after which a further addition ofwater continuously increases the process which acceleratesautocatalytically. As a consequence, the temperature rises beyond 170 C.and preferably above 200 C. and thus creates the conditions forformation of the end products Xonotlite (calcium hydrosilicate) andCerolite (magnesium hydrosilicate). The amount of water used therefore,for practical purposes, will be based on the course of the reaction andits pressure and temperature conditions.

The hydrothermal conversion is thus advantageously carried out attemperatures above 170 C. and preferably above 200 C. As startingmaterials there may be used caustic calcined dolomite and pit quartzsand or other suitable SiO carriers, together with the disclosedquantities of water. The calcined dolomite should contain at least CaOand MgO and the mole ratio of CaO to MgO should effectively amount toabout 1:1. A slight deviation from this mole ratio, as it frequentlyoccurs in natural dolomite, however is not critical. For practicalpurposes, the quartz sand should be used in the form of a fine grain, asa result of which the conversion is accelerated.

The hydrothermal conversion reaction can be carried out in theconventional manner used, for example, in the hardening of limesandstone and can also be carried out in autoclaves customary for thispurpose. According to a preferred variation of the process according tothe present invention, however, the hydrothermal conversion is carriedout with a simultaneous grinding or crushing of the reaction mixture. Inthis way, a far more rapid reaction takes place to form thehydrosilicates.

The comminution of the reaction mixture through grinding is effectivelyaccomplished according to the principle of autogenic grinding, wherebythe grains of quartz and dolomite will comrninute each other. At thesame time, the hydrosilicates formed on the surfaces of the grains arescoured and new surfaces are opened. According to the process of thepresent invention therefore, one can use standard commercial quartz andcalcined dolomite grains, and it is not necessary to start withexpensive powders. After the conversion: 6(CaO'M g0) 12 SiO,+7H OQCaO-6Si0 H O+6(MgO-SiO2 1-1 an intimate mix ture of the calciumhydrosilicate Xonotlite and the magnesium hydrosilicate Cerolite isformed.

Referring now more particularly to a detailed description of a preferredmode of the invention for the production and further treatment ofcalcium and magnesium hydrosilicates, the SiO, component, for examplequartz sand, and calcined dolomite are fed at a desired mixing ratio,e.g. 1:1, into the reaction vessel. The autoclave mill" preferably usedin the present invention is an enclosed vessel which has been modifiedto be rotatable and thus also function as a grinding apparatus. Afterthe charge, the autoclave mill is closed, put in rotation and 10-20percent of the required quantity of water is measured and fed in. Thetemperature of the reaction mixture rises and further water is measuredin in accordance with the rise in temperature. The reaction is concludedwhenever the required quantity of water has been added and a drop intemperature becomes noticeable. The autoclave mill is emptied into oneor several flash containers into which the reaction product is forced bythe pressure existing in the mill. Any remaining reaction product isremoved from the autoclave -mill. After separation of solid substancesfrom the reaction product, the remaining product may be briquetted moistand, corresponding to the subsequent treatment desired, fed into atunnel kiln or into a chamber furnace and converted to diopside bycalcining, e.g. at a temperature between 1,200 and 1,250 C. In thealternative, the reaction product may be introduced as a sludge into arotary kiln wherein it is calcined into diopside.

The dolomite used herein preferably contains at least 40 percent of theCaO component. It is to be understood that instead of calcined dolomite,one could also convert dolomite hydrate with silicic acid; however, itis preferable to utilize the heat developed during slaking of the CaOcomponent to heat up the dolomite-silicic acid mixture for thehydrothermal conversion reaction, especially when said reaction isaccompanied with simultaneous grinding and crushing of the reactionmixture. Therefore, preferably, the dry reactants are first placed intoan autoclave mill, preferably already containing some previously groundmaterial, and the required quantity of water is then added graduallywhile simultaneously grinding the reaction mixture.

The following are specific examples of preferred embodimerits of thepresent invention.

EXAMPLE 1 Substantially equivalent weight amounts of calcined dolomiteand quartz sand were charged into an autoclave mill. The autoclave millwas closed and rotated and 15 percent of water, based on the combinedweight of dolomite and silicic acid, was fed into the mill. Thetemperature of the reaction mixture rose to above 200 C. and additionalwater was added to the mill. As soon as the temperature in the autoclavemill began to drop, addition of water was ceased. The calcium andmagnesium hydrosilicate reaction product was transferred to amulticyclone wherein solid substances were removed.

A portion of the hydrosilicates obtained was briquetted. Half of thebriquettes were placed in a tunnel kiln and the other half into achamber furnace wherein the temperature was raised to about 1,200" C.and the hydrosilicate mixture was converted into diopside.

Another portion of the hydrosilicates obtained was transferred into arotary kiln. The temperature therein was raised to about 1,250 C. andthe hydrosilicate mixture was converted into diopside. In bothinstances, the calcined product had a specific weight of 3.3 g./cc. anda melting point of about 1 ,390 C.

What is claimed is:

1. A process for the production of synthetic diopside which compriseshydrothermally converting dolomite and silicic acid into syntheticXonotlite and Cerolite by mixing the dolomite and silicic acid andgradually adding thereto from 10 to 40 percent by weight of water basedon the combined weight of the dolomite and silici c acid in a closedreaction vessel until the temperature in said vessel reaches at leastC., and

thereafter dehydrating said Xonotlite and Cerolite at a temperature offrom l,200 to l,250 C. to form said diopside.

2. The process according to claim 1 wherein the silicic acid is providedin the form of quartz sand.

3. The process according to claim 1 wherein the hydrothermal conversionis conducted at a temperature of at least 200 C.

4. The process according to claim 1 wherein the hydrothermal conversionis conducted while simultaneously comminuting the reaction mixture.

5. The process according to claim 4 wherein comminution of the reactionmixture is an autogenic comminution.

i i i t

2. The process according to claim 1 wherein the silicic acid is providedin the form of quartz sand.
 3. The process according to claim 1 whereinthe hydrothermal conversion is conducted at a temperature of at least200* C.
 4. The process according to claim 1 wherein the hydrothermalconversion is conducted while simultaneously comminuting the reactionmixture.
 5. The process according to claim 4 wherein comminution of thereaction mixture is an autogenic comminution.